Currently we are using MariaDB for most database needs. The main reason we've continued to use MariaDB is for the multi-master replication. It is simple and easy to setup, we've designed our applications assuming there is always a write master, but aware of possible slag lag. So far having a multi-master setup is not as simple or robust in PostgresSQL.

We also use tokudb storage engine for a large number of servers. This engine is designed for datasets that do not fit into memory, and we've had a great experience with it. We are able to load CSV's in 1/3 to 1/5th the time of innodb. Most queries aggregate queries run faster, there is online index creation and column changes. From a DBA prospective tokudb prints detailed statuses in the show processlist! It is definitely a niche storage engine, and not right for all cases but this has contributed to our continued use of MariaDB of PostgreSQL.

With PostgreSQL 10 things are looking interesting. We tested out 9.6 with it's parallel query and saw some significant improvements on aggregation queries. It was not enough by itself to warrant use moving off of MariaDB but has me very excited going forward.

MariaDB is not sitting still, there are many improvements coming to 10.2 which should be released soon. For most applications I'd use whatever database you are familiar with. If you have a specific case, terabytes of data, millions of incoming writes, requirements for live aggregation, that is where the PostgreSQL vs MariaDB really gets exciting.